The natural intervertebral disc contains a jelly-like nucleus pulposus surrounded by a fibrous annulus fibrosis. Under an axial load, the nucleus pulposus compresses and radially transfers that load to the annulus fibrosis. The laminated nature of the annulus fibrosis provides it with a high tensile strength and so allows it to expand radially in response to this transferred load.
In a healthy intervertebral disc, cells within the nucleus pulposus produce an extracellular matrix (ECM) containing a high percentage of proteoglycans. These proteoglycans contain sulfated functional groups that retain water, thereby providing the nucleus pulposus with its cushioning qualities. These nucleus pulposus cells may also secrete small amounts of cytokines as well as matrix metalloproteinases (MMPs). These cytokines and MMPs help regulate the metabolism of the nucleus pulposus cells.
In some instances of degenerative disc disease (DDD), gradual degeneration of the intervertebral disc is caused by mechanical instabilities in other portions of the spine. In these instances, increased loads and pressures on the nucleus pulposus cause the cells within the disc (or invading macrophages) to emit larger than normal amounts of the above-mentioned cytokines. In other instances of DDD, genetic factors or apoptosis can also cause the cells within the nucleus pulposus to emit toxic amounts of these cytokines and MMPs. In some instances, the pumping action of the disc may malfunction (due to, for example, a decrease in the proteoglycan concentration within the nucleus pulposus), thereby retarding the flow of nutrients into the disc as well as the flow of waste products out of the disc. This reduced capacity to eliminate waste may result in the accumulation of high levels of proinflammatory cytokines and/or MMPs that may cause nerve irritation and pain.
As DDD progresses, toxic levels of the cytokines and MMPs present in the nucleus pulposus begin to degrade the extracellular matrix. In particular, the MMPs (as mediated by the cytokines) begin cleaving the water-retaining portions of the proteoglycans, thereby reducing their water-retaining capabilities. This degradation leads to a less flexible nucleus pulposus, and so changes the loading pattern within the disc, thereby possibly causing delamination of the annulus fibrosis. These changes cause more mechanical instability, thereby causing the cells to emit even more cytokines, typically thereby upregulating MMPs. As this destructive cascade continues and DDD further progresses, the disc begins to bulge (“a herniated disc”), and then ultimately ruptures, causing the nucleus pulposus to contact the spinal cord and produce pain.
One proposed method of managing these problems is to remove the problematic disc and replace it with a porous device that restores disc height and allows for bone growth therethrough for the fusion of the adjacent vertebrae. These devices are commonly called “fusion devices”.
Designs of intervertebral fusion devices are generally either box-like (i.e., Smith-Robinson style) or threaded cylinders (i.e., Cloward style). Smith-Robinson style implants have the advantage of better contact area to the endplates, but rely on a coarse surface texture or teeth to prevent migration once implanted. Insertion then requires over distraction of the disc space to slide the implant in or to provide a smoother implant, which can migrate post-op.
One such box-like design is the Brantigan cage. U.S. Pat. No. 4,743,256 (“Brantigan”) discloses an improved surgical method for eliminating spinal back pain caused by ruptured or degenerated vertebral discs by spanning the disc space between adjacent vertebrae with rigid fusion devices, or “cages”, having surfaces facilitating bone ingrowth and bottomed on prepared sites of the vertebrae to integrate the implant with the vertebrae and to provide a permanent weight supporting strut maintaining the disc space.
One commercial box-like design is the injection-molded carbon fiber reinforced PEEK (CFRP) cage made by DePuy Spine. However, these cages are difficult to insert because of the interference fit produced between the textured, toothed upper and lower surfaces of the implant and the bony endplates. Simply, the presence of teeth extending from the upper and lower surfaces of the cage make its insertion difficult. In addition, the reinforced PEEK material is brittle and so is prone to breakage when applying impact or torque loads to the implant to overcome tooth-induced resistance during insertion and final positioning of the implant.
Current interbody devices are made from single materials (e.g., machined titanium, or molded and/or machined PEEK). Titanium has the disadvantage of being radiopaque (which can interfere with fusion assessment on x-ray) while also having a high modulus of elasticity (which can stress shield the bone graft). Injection molded CFRP is very brittle and prone to fracture during insertion. Unfilled PEEK is much less brittle but also weaker than carbon-filled PEEK, requiring thicker-walled designs (diminishing space for bone graft). Both PEEK and carbon-filled PEEK are radiolucent.
U.S. Pat. No. 6,761,738 (“Boyd”) discloses a modular intervertebral spacer formed from assembled bone-derived components. In particular, Boyd discloses an assembly of vertical planks with cylindrical cross pins. However, Boyd does not disclose the use of non-allograft materials of construction, any companion instrumentation for insertion of the device, nor a method of placing the device first into the disc space and then filling it with a biologic material Allograft bone is very brittle, and so it is difficult to securely join such pieces together.
U.S. Pat. No. 6,413,278 and U.S. Pat. No. 6,835,208 (“Marchosky”) disclose an I-beam shaped implant whose top and bottom surface flex under anatomic loads. There is no interior space to this implant, and Marchosky teaches that two such implants need be implanted in a single disc space. Although a syringe for injecting bone graft around the implant is disclosed, a mating inserter instrument for placing the implant is not disclosed.
U.S. Pat. No. 6,824,565 (“Muhana”) discloses implant and instrument designs wherein some of the implant embodiments have planked designs and a mating inserter instrument. However, the disclosed inserter wraps around the exterior of the implant and partially into grooves on the implant. The disclosed implant is derived from bone and is not hollow. The insertion technique disclosed by Muhana requires a cutting tool to prepare a channel for the implant.
US2005/0209696 (Lin) discloses an intervertebral implant system for intervertebral implantation, wherein the system includes a frame having a peripheral wall defining a space therein, and a settable material introducible into the space of the frame. The settable material is a biocompatible load bearing material including and not limited to bone, composites, polymers of bone growth material, collagen, and insoluble collagen derivatives. The settable material is injectable into the space defined by the frame. The settable material may have an initial fluid condition wherein the fluid settable material cures to a hardened condition. Lin further includes the steps of accessing the disc space between adjacent intervertebral discs; removing disc material from the disc space; distracting the disc space; preparing the end plates of the adjacent intervertebral discs; inserting the peripheral wall of the frame into the disc space between the adjacent intervertebral discs; and injecting settable material into the space defines by the peripheral wall of the frame and between the adjacent intervertebral discs. Lin teaches that the method may further include the step of connecting each free end of the peripheral wall to one another. Lin teaches that the method further includes the steps of inserting a plurality of frames into the disc space between the adjacent intervertebral discs, wherein each frame defines a space; and injecting settable material into at least one of the spaces defined by the frames.
In summary, the insertion of both smooth and toothed intervertebral cages has proven to be problematic. Whereas toothed cages are difficult to insert, cages with smooth upper and lower surfaces have demonstrated undesirable migration.